Antennas mounted on high altitude platforms (HAPs) can be used to implement a cellular system for wireless communications with users on the ground from a base station located at high altitude. HAPs, such as blimps or aerial vehicles, have several limitations which make designing an antenna for cellular system challenging. First, since most HAPs obtain their required power from solar energy, they have very limited power available for all of the onboard communication equipment such as the base-band units (BBUs), backhaul systems, antennas, as well as other mechanical and monitoring equipment. Second, HAPs are typically installed at more than 20 km above the ground and the signal transmitted from a HAP to the users on the ground can experience large path losses. Third, HAPs are very costly to build. Therefore, to make a HAP-based cellular wireless network economically feasible, the number of HAPs that are deployed should be as few as possible for providing the desired coverage over the entire target region.
Since the path losses between the base station and users is significant and the available transmit power is quite limited, large phased array antennas or antennas with large antenna gain (such as horn antennas or dish antennas) are typically used for HAP wireless communication applications. The higher antenna gains of such antenna systems are used to compensate for the larger path losses. The large phased array is more suitable for HAP cellular applications, as multiple narrow beams can be generated by a single antenna panel for multiple cells, thereby increasing network throughput without increasing deployment cost.
Traditionally, planar phased array antennas have been proposed for HAP cellular systems where the panel is installed underneath the HAP and is used to create multiple narrow beams. In that case, a cell is created on the ground by generating a beam (or a pair of beams in the case of cross-polarized elements). FIG. 1 shows the schematic representation of a HAP 1 with a planar array 2 installed underneath of the HAP. The normal of the panel is perpendicular to the HAP and vertical to the ground (e.g. the panel is parallel to the ground). The planar array 2 is used to generate two beams 3 and 4 which represent two cells on the ground. Similar to terrestrial networks, each cell is controlled by a BBU (not shown) and mobile users need to be handed over from one cell to another depending on which cell provides the best signal. The size of the array and the number of beams that are generated by the array are designed based on user density and required network capacity and coverage. For instance, more cells (more narrow beams) provide higher capacity with the consequence of requiring a larger array to create more narrow beams and to avoid inter-beam interference.
FIG. 2 shows the signal-to-interference-plus-noise ratio (SINR) contour of a HAP-based cellular system using a 14×14 planar cross-polarized array. The HAP is located at 20 km above the ground. Nineteen cells with intercell distance (ICD) of 6 km are created by forming 19 narrow beams with cross polarization diversity (38 beams in total). The coverage region of this panel is an area with a radius of 13 km. FIG. 2 shows that the cell at the center which is located at the boresight of the antenna has circular shape and as one moves away from the boresight, the beam shapes become asymmetric due to the increasing scan angle of the panel. The intercell distance can be adjusted to achieve the required coverage and capacity to be delivered by HAP. For a specific array size, there is usually an optimal ICD which delivers the maximum capacity. The optimal ICD can be found using intensive simulations. FIG. 3 shows the simulation results of the capacity performance of a 14×14 planar array versus ICD. The optimal capacity for this specific panel is achieved at ICD of 6 km.
The number of cells (i.e., beams) and quality of signal in each cell determine the capacity that can be delivered by a HAP. The capacity of the covered area can be increased by increasing the number of cells. However, there is a trade-off since the inter-beam interference increases with an increasing number of cells. Thus, the number of cells cannot be increased without decreasing the beamwidth of each beam, which can be done by using a larger array.
While the capacity of the HAP can be improved by using a larger array and a higher number of cells, improving the coverage region is difficult. One can argue that the coverage region can be increased by increasing ICD. However, increasing ICD creates outage areas in the network. FIG. 4 shows the simulation results for the outage performance of a HAP-based cellular system using a 14×14 cross-polarized planar array. Outage is defined as the percentage of the covered area receiving SINR of −5 dB or lower. It can be seen that as ICD increases, the outage grows rapidly in the network. Therefore, for this specific antenna array, the beams cannot be placed further than 8 km apart to maintain a reasonable amount of outage. This is mainly because each cell is generated by a narrow beam and the narrow beam of a 14×14 planar array has a very sharp roll-off, meaning that the antenna gain decreases rapidly as one moves away from the boresight of the beam, as shown in FIG. 5. At a large ICD, a narrow beam cannot cover the entire region inside the cell with good signal quality, creating a large amount of outage. This is illustrated in FIG. 6, which shows the SINR contour of 14×14 panel with an ICD of 10 km. Notice that large areas between beams are not covered with good signal quality. One can decrease the size of panel to create wider beams for larger coverage; however, as the size of the panel decreases, the antenna gain decreases as well, and thus signal strength for users on the ground will suffer.